How to Make Helix antenna by kilodelta55


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									Helical/helix antenna cookbook recipe for 2.4
GHz wavelans and/or WiFi applications

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this cookbook recipe and placed it on my local
ADSL-connected machine, never expecting that so many of you want to
have this information.
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The helix antenna, invented in the late fourties by John Kraus (W8JK),
can be considered as the genious ultimate simplicity as far as antenna
design is concerned. Especially for frequencies in the range 2 - 5 GHz
this design is very easy, practical, and, non critical. This contribution
describes how to produce a helix antenna for frequencies around 2.4
GHz which can be used for e.g. high speed packet radio (S5-PSK, 1.288
Mbit/s), 2.4 GHz wavelans, and, amateur satellite (AO40).
Developments in wavelan equipment result in easy possibilities for high
speed wireless internet access using the 802.11b (aka WiFi) standard.

Theory in a birds eye view
The helix antenna can be considered as a spring with N turns with a
reflector. The circumference (C) of a turn is approximately one
wavelength (l), and, the distance (d) between the turns is approx. 0.25C.
The size of the reflector (R) is equal to C or l, and can be a circle or a
square. The design yields circular polarization (CP), which can be either
'right hand' or 'left hand' (RHCP or LHCP respectively), depending upon
how the helix is wound. To have maximum transfer of energy, both ends
of the link must use the same polarization, unless you use a (passive)
reflector in the radio path.
The gain (G) of the antenna, relative to an isotrope (dBi), can be
estimated by:
G = 11.8 + 10 * log {(C/l)^2 * N * d}
dBi               (1)

According to Dr. Darrel Emerson (AA7FV) of the National Radio
Astronomy Observatory, the results from [1], also known as the 'Kraus
formula', are 4 - 5 dB too optimistic. Dr. Ray Cross (WK0O) inserted
the results from Emerson in an antenna analysis program called 'ASAP'.

The characteristic impedance (Z) of the resulting 'transmission line'
empirically seems to be:

Z = 140 * (C/l)
Ohm                                                          (2)

Practical design for 2.43 GHz (aka S-band, ISM band, 13 cm
amateur band)

l = (0.3/2.43) = 0.1234567 m                  ;-)(12.34
cm)             (3)

The diameter (D) of one turn = (l/pi) = 39.3
mm         (4)

Standard PVC sewer pipe with an outer diameter of 40 mm is perfect for
the job and can be obtained easily (at least in The Netherlands ;-) from a
'do it yourself' shop or a plumber. The helix will be wound with standard
wire used to interconnect 220V AC outlets in (Dutch ;-) house holds.
This wire has a colourized PVC isolation and a 1.5 mm thick copper
core. Winding it around the PVC pipe will result in D = ca. 42 mm, due
to the thickness of the isolation.

With D = 42 mm, C = 42*pi = 132 mm (which is
1.07 l)    (5)

Now d = 0.25C = 0.25*132 = 33
mm                        (6)

For distances ranging from 100 m - 2.5 km with line of sight, 12 turns
(N = 12) are sufficient. The length of the PVC pipe therefore will be 40
cm (3.24 l). Turn the wire around the PVC pipe and glue it with PVC
glue or any other glue containing tetrahydrofurane (THF). The result will
be a very solid helix wound along the pipe, see figure 1 below.

Figure 1. Overview of some of the materials used and dimensions.

The impedance of the antenna, which is:

Z = 140 * (C/l) = 140*{(42*pi)/123.4} = 150
Ohm       (7)

requires a matching network on order to apply standard 50 Ohm
UHF/SHF coax and connectors.
The use of a 1/4-wave matching stub with an impedance (Zs) of :

Zs = sqrt(Z1*Z2) = sqrt(50*150) = 87
Ohm                (8)

is very common. Due to the helix design, this equals 1/4 turn. However,
from a mechanical point of view -bearing water proof aspects in mind
when using the antenna outdoors- there are more preferred methods to
match the helix to 50 Ohm. My first thoughts were to empirically
decrease d for the first and second turn and match the helix using the
'trial and error'-method, while measuring the results with a directional
coupler, and signal generator. Browsing the internet for while I found
helices matched this way, but surprisingly I bumped into the page
of Jason Hecker. He really used an elegant way to match his helix by
using a copper vane, referring to the ARRL Handbook. So, full credits
go to the ARRL and Jason, and I used his dimensions for the vane. To be
honest, this page seems to be a duplicate of his page, except that our
helices are wound the other way around!! Yes, and I am left handed, so,
is this a coincidence? It is funny anyway :-)) For details, see figure 2
Figures 2a and 2b. The idea, the dimensions, and, mounting the stub.
The hypotenusa of the stub should follow the wire.

Now with some luck and skills solder the stub to the helix, glue it, and
prepare the contrapsion to be inserted into
the cap, see figure 3.
Figure 3. Almost finished helix antenna.

And.... ready! (figure 4)

Figure 4. Finished 12 turn 2.4 GHz helix antenna, G = 17.5 dBi or 13.4
dBi (Kraus or Emerson respectively)

The antenna was sweeped an measured. The results are given below
(figures 5a and 5b)
Figure 5a Return loss (dB) from 2300 - 2500 MHz   Figure
5b Smith chart 2300 - 2500 MHz
Figure 6a Measurement setup                    Figure 6b 'helix-in-
one-hour' and Rohde & Schwarz analyser

And... finally.... the helix 'in action'....
Figure 7a Beaming to my LAP (Local Access Point ;-
)             Figure 7b 'bottom view'

It is really nice to receive feedback from people who are inspired by this
page. Here a contribution from Rob Jaspers who made
his helices using this page:

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